Assiqnob to general cable



INSULATION Filed Jan. 9, 1929 N11-N555 lNvENToR Patented Sept. 2, 1930 UNITED STATES PATENT OFFICE DONALD M. SIMMONS, F OSBORNE, PENNSYLVANIA, ASSIGNOR T0 GENERAL CABLE CORPORATION, OF NEW YORK, N. Y., A CORPORATION 0F NEW JERSEY INSULATION Application led January 9, 1929. Serial No. 331,171.

My invention relates to improvements in insulation for electric conductors, and particularly to insulation which includes laminael of paper, cloth, or other porous material,

filled with insulating substance. The insulating substance when applied is in liquid condition. A typical case is cable insulation formed of paper tape wrapped to place and filled with insulating compound. The object 1o of my improvements is to strengthen such a body of insulation against break-down.

The accompanying drawings afford illustration. Fig. I is a fragmentary view of a cable, showing in elevation the conductor itself, and diagrammatically and in axial section a body of surrounding insulation built in embodiment of my invention. Fig. II is a View in perspective of a tape employed in the building of the body of insulation of Fig. I. Figs. III and IV are views similar to Fig.

I and illust-rating certain modifications.

The paper or other porous material which forms the immobile portion of such an insulating envelope as I have indicated is ordinarily applied in the form ,of a ribbon or tape, and in the building up of the laminae one upon another small spaces are commonly formed between the successive turns of tape in any given layer; for, ordinarily, in the laying of the tape the adjacent edges of successive turns are spaced apart at a slight interval. In the building of cables particularly, the tape is laid spirally around the conductor, and the spacing apart of the edges of successive turns is a feature of value and importance, since it affords in the finished cable superior flexibility. The cable may be bent without undesired wrinkling of the paper of which thebody of insulation is fundamentally composed. In the building of layer upon layer the spaces between succeeding turns in an underlying layer are covered by the continuous web of the next overlying layer. That is to say, the successive layers of paper or its equivalent are so superposed that the spaces in one layer are offset with respect to those of an adJacent layer. This is illustrated in the accompanying drawings. In Fig. I the cable conductor is indicated at l. 50 Upon it the tape 2 is spirally wrapped, and

between the turns of the spiral spaces a, b, c, d, e, are left. The spaces b of the second layer are offset with respect to spaces a of the first layer; and the lspaces c of the third layer are offset with respect to the spaces b of the second; and so on.

These spaces, indeed, are of small size.

age every weakness is searched out, and it i becomes a' matter of engineering to design a cable of minimum dimensions and cost to carry, through long periods of time, current of given maximum power.

As between a body of paper or equivalent fibrous material filled with oil or other fluid or viscid insulation and a free body of such fiuid or viscid insulation of like dimensions, the former is, dielectrically considered, much the stronger. And, with attention confined to free bodies of liquid or viscid insulation, the unit dielectric strength is much greater, relatively speaking, in a thin layer than in a layer of greater thickness. In structures such as thosek under consideration, all bodies of compound which fill spaces otherwise void are weaker than the bodies of compound-filled paper, but the bodies of compound which fill the spaces a, b, c, etc., since they are of much greater radial depth than any other bodies of free compound within the structure, are by far the weakest portions of the Whole insulating envelope.

As cables such as those shown in the drawings continue in use, there is apt to be diminution in compound-whether because of further and more complete absorption by the fibrous bodies, or in consequence of flow to other regions-and consequent voids are apt to a pear. And voids, wherever they occur, are, eyond all else, weaker.

In the transmission of alternating current the stress upon the insulation is distributed in inverse proportion to relative specific inductive capacity. That is to say, at points where the specific inductive capacity of the insulating wall is less, the strain tending to break-down is greater.

Coming to conditions of actual practice, the body of compound-filled porous paper may have a specific inductive capacity of about 3.5; whereas free bodies of liquid or viscid insulation if present within the spaces a, b, c, etc. may have a specific inductive capacity of 2 or slightly more than 2; and voids, if voids are found, have a specific inductive capacity of 1.

Therefore the tendency to break-down through the oil-filled spaces a, b, c, etc. (if, indeed, these spaces be filled with oil) which exists because of the inferior strength of free bodies of oil toresist break-down, is intensified, because of t-he concentration of stress upon these regions of low specific inductive capacity. And in the use of cables whose insulation is of the character described, whether the current carried be direct or alternating, it is found that the cable tends to break down precisely at the compound-filled spaces. It is there that, under excessive stress, ionization occurs and charring begins, accompanied sometimes and perhaps facilitated by chemical change in the insulating compound itself.

My invention consists in a structure in which the bodies of compound in these interstitial spaces are relieved of stress, and the structure of my invention includes bodies of conducting material isolated within the body of insulation and preferably affording to the compound within such interstitial spaces opposite limiting surfaces, such limiting surfaces bein`g spaced apart in the direction of stress. The bodies of conducting material which so limit and define bodies of insulating compound are electrically connected; and thus the limited body of compound is relieved of stress when the structure is in service.

The invention as it is illustrated in Figs. I and II is found in a conducting facing of the inner and outer walls and one of the side walls of each such interstitial space.

' As a matter of practice, I find it convenient to metallize the paper tape of which the immobile portion of the insulating envelope is built along one edge and over areas upon upper and lower surfaces-areas of such restricted width, however, that while in the assembly the spaces are defined and formed between inner and outer walls of metal, there is no other continuity between the metallized areas of successive layers, than that desired continuity which the metallized edge of `he tape affords between the limiting walls of each space.

In Fig. II the taper is diagrammatically illustrated, and is shown to be metallized over portions 3 and 5 of its opposite surfaces, less in extent than the width of the tape, and over one edge 4. The tape so superficially metallized is spiralled in successive layers about the conductor to form the structure of Fig. I. And all the body of insulation, in-

cluding the spaces, `is filled with compound. In Fig. I it will be seen that each space a, b', c, al, (and space e as well, when eventually the cable sheath is applied) is bounded by inner and outer walls of metal, and that these walls are made electrically continuous by contact of the areas of metal which form these walls with the metal which covers one edge of the tape.

These bodies of metal isolated in the depth of the whole body of insulation have no appreciable effect, to diminish the dielectric character of the whole body of insulation; and yet they have effect to relieve the compound-filled spaces of strain, and so lto strengthen the structure electrically at its weakest points. Other things being equal, a cable in which my invention is embodied ma be made smaller without loss of strength, or, if built t0 the saine dimensions, will be stronger to resist break-down than a cable which lacks the invention.

I have described metallized areas upon vthe edge and upon the faces of the tape. These areas need not be highly conducting throughout all their extent. The conducting material may be painted on or rubbed on, as With a pencil, or it may be sprayed on. It may be applied as a foil, or as a self-sus taining strip, gathered with the other parts in the building of the envelope of insulation. It may be of metal, or of other suitable conducting material-for instance, graphite. I contemplate the assembly of materials having the described characteristics, and have indicated various practical ways in which the assembly may be effected. The invention in broader aspect is found, not in the manner in which assembly is effected, but in the assembly itself.

The invention may be practiced with less precision Iin structural detail, but with effect, in the manner illustrated in Fig. IV. In this case, as the tape of paper or equivalent material is spiraled around the conductor, strands 6 of metal wool are laid in the spaces a, b, c, etc., between the successive turns of tape. The body of metal wool so lying within the space accomplishes the same end which the space walls faced with conducting material already described accomplish. The strands of metal extending within the space subdivide the space, and'make electrical contact one with another at short intervals. The'consequence and effect are that the otherwise free body of compound is penetrated by interlaced strands of metal. These strands are, by their frequent contacts, bodies of equal potential when the cable is in service; and, being of equal potential, and being spaced apart (many of them) in the direction of stress, they relieve the intervening volumes of compound of such stress. Since the strand of metal wool is isolated within the body of insulation it has no substantial effect, to diminish the security of the insulation as a whole.

The structure of Fig. III differs from that of F ig. I in two respects: rst, in that the successive layers are applied with an overlay which brings the space, not to the middle of the underlying and overlying strips, but to a point one third of the way across the width of the strip. This is a characteristic of oommon practice. To adapt the invention to such spacing it is necessary to arrange the areas 3 and 5 not in opposite positions, but each one third of the way, one from one edge the, other from the other edge of the strip. The second feature which distinguishes the structure of Fig. III from that of Fig. I is that the spaces, a, b, c, etc., in addition to being walled about with conducting material contain also metal wool, as in the structure of Fig. IV. Thus the specilic forms of the invention severally shown in Figs. I and IV are in Fig. III shown to be combined. And, being combined, they manifestly unite in accomplishing the described effect of relieving the otherwise weak portion of the body of insulation of stress tending to destruction. Apart from other considerations itis manifest that, should there be such displacement of the component tapes in the structure of Fig. III as to interrupt the electrical continuity of the outer and inner walls of a given space a, b, c, etc., still the filling of metal wool will be effective, the end in view.

I claim as my invention:

l. In an electric cable structure a body of wrapped-on porous insulation filled with insulating compound and including spaces between longitudinally succeeding turns of the wrapped-on insulation filled with such compound, bodies of conducting-material isolated within the body of insulation and affording opposite surfaces to volumes of insulating compound within such spaces, such bodies being electrically connected.

2. In an electric cable structure a conductor, a body of wrapped-on porous insulation enveloping said conductor with interstitial spaces between longitudinally succeeding turns of such wrapped-on insulation, a body of insulating compound filling such porous insulation and the interstitial spaces included therein, the limiting walls of such interstitial spaces being formed of conducting material isolated within the body of insulation, such opposite walls of each space being electricallyconnected.

3. In an electrical installation in combination with insulation consisting of a body of to accomplish porous material iilled with insulating compound, and including bodies of metal wool isolated within spaces in the body of porous material.

4. In an electric cable structure a conductor, a body of tape of insulating material provided with an incomplete surface coating of conducting material wrapped upon the conductor forming of itself alone an envelope with interstitial spaces, the opposite inner and outer walls and one side wall of each space being formed and dened by the said coating of conducting material upon the constituent tape, and a body of insulating compound filling the envelope and the interstitial spaces. 5. In an electrical cable structure a conductor, a body of wrapped-on porous insulating material enveloping the conductor, the continuity of the said body of insulating material being interrupted by spaces whose inner and outer walls are metal faced, bodies of metal wool isolated within such spaces, and insulating compound filling the whole.

In testimony whereof I have hereunto set my hand.

DONALD M. SIMMONS. 

